Thyroid hormones regulate a very large spectrum of important biological processes including development, metamorphosis, growth, differentiation, metabolism, and homeostasis. A central question has been how this large array of programs can be regulated by a limited number of hormones and how individual-responses can be restricted to certain cell types and developmental stages. A detailed understanding of these mechanisms offers elucidation of biological programs and optimal usage of these hormones and their analogs in therapies. Like the steroid hormones thyroid hormone (T3) signals are mediated by nuclear receptors that belong to one of the largest transcription factor families known today. The complexity of the thyroid hormone response is displayed to some degree at the receptor level by two genes (TRalpha and TRbeta) that give rise to multiple isoforms, some of which are not ligand dependent transcriptional activators. Until recently it had been assumed that TRs, like the steroid hormone receptors mainly function by one "direct" pathway that involves binding of the receptors as homodimers to specific DNA sequences. However, during the last two years our view on how the receptors operate has changed dramatically. Recent results strongly suggest that TRs require heterodimerization with Retinoid X Receptors (RXRs) for effective DNA binding and function. Furthermore, TRs have been found to regulate gene transcription by a novel "indirect" mechanism where TRs interact with the transcription factor AP-l. AP-l mediates signal transduction by growth factors, oncogenes, and the tumor promoter TPA. By interfering with this pathway, TRs can function as anti-oncogenes. In the coming years we will further analyze the basic mechanisms of TR action to obtain a basis for understanding thyroid hormone action in the normal and disease states. We will investigate the roles of TR homodimers and heterodimers in the direct and indirect response pathways. The role and function of TR carboxy terminal isoforms will also be analyzed employing gene knockout technology and by identifying response elements and co-receptors for these isoforms. To further understand on how TRs mediate crosstalk between thyroid hormones and AP- l mediated signalling, studies analyzing the molecular mechanism of TR-AP-1 interaction will be carried out. The proposed research will further our understanding of gene regulation and pathways controlled by TRs. These results will enhance our knowledge of T3 action in normal physiological processes and disease.